Field of the Invention
The present invention relates to an antireflection film provided to an optical element.
Description of the Related Art
A conventional optical element is provided with, on its surface, an antireflection film to reduce loss in a light quantity of incident light. For example, an optical element for visible light is provided with a dielectric multi-layered (multi-coated) film. A known antireflection film has a fine structure.
Japanese Patent Laid-open No. 2012-78597 discloses an antireflection film including, as a top layer, a layer formed of a nanoporous film or a nanoparticle film and having a low refractive index. Japanese Patent Laid-open No. 2012-78597 discloses that the antireflection film formed to have only a film thickness (100 nm or larger) allowing an accurate formation exhibits a small performance degradation due to a film thickness variation generated in manufacturing. Japanese Patent Laid-open No. 2008-233880 discloses an antireflection film having an effective refractive index that continuously changes from a light incident side to a substrate side and including a top layer formed of plate crystal including aluminum oxide as a primary component.
Japanese Patent Laid-open No. 2012-78597 discloses that a high performance is obtained for vertically incident light. However, since thin film interference is exploited to prevent reflection, reflection-preventive performance is expected to largely degrade for obliquely incident light, especially for light having an incident angle of 45 degrees or more, for which an interference condition is not satisfied. In addition, forming a film on a lens having a large opening angle has difficulties in providing a uniform film across a peripheral part and a central part of the lens. Optimizing a film thickness at the central part is likely to result in a degraded reflectance at the peripheral part. On the other hand, the prior art disclosed in Japanese Patent Laid-open No. 2008-233880 does not disclose detailed designed values (refractive index structure) and antireflection characteristics of films.